The term heavy is used, with exceptions noted below, during all radio transmissions between air traffic control and any aircraft which has been assigned a maximum takeoff weight (MTOW) rating of 136 tonnes (300,000 lb) or more. Aircraft with a MTOW rating between 7 t and 136 t are considered medium and aircraft with a MTOW rating less than 7 t are considered light. In the US, the FAA uses a slightly different categorization. Aircraft capable of maximum takeoff weights more than 41,000 pounds (19 t) and less than 300,000 pounds (140 t) are considered large.
Such "heavy" aircraft over 136 t create wake turbulence from the wings. The term is mostly used around airports during take off and landing, incorporated into their call sign so as to warn other aircraft that they need extra distance to avoid this wake turbulence. All wide-body aircraft are classified as heavy, with the exception of the Airbus A300B1 (MTOW of 291,000 pounds (132 t), only two built and retired in 1990), the Airbus A380, and the Antonov An-225, which are classified in the even larger category of super. Certain variants of the narrow-bodied Boeing 707 and Douglas DC-8 were considered "heavy" based on MTOW.
Of special note here is the narrow-bodied Boeing 757. Under current guidance, the 757 is considered large, as it has a maximum MTOW of 116,000 kilograms (256,000 lb). After a number of accidents where smaller aircraft following closely behind a 757 crashed, however, the rules were changed so that controllers are required to apply the special wake turbulence separation criteria specified in paragraph 5-5-4 in the FAA guidelines for aircraft separation, as though the 757 were heavy.